In the previous article, we got a glance at the telescope that we'll be using in our observatory. We have a mount, a temporary tube and the next step will be to choose a CCD camera for the observation.
Choosing a CCD camera is a multi-factor problem. So, depending on where you place the cursor of your needs the answer may vary. My purpose here is to introduce some concepts specific to sky observation. I will really try to make it short and not go through all the aspects of CCD sensors. Just look at the references section at the bottom of this post. In the noise article from QSI, you will see that you can write books on only one subject.
You may know a lot about cameras for shooting films in standard earth environment and you may even feel comfortable in dramatizing the next “Blair witch project”, but because of light conditions, usual cameras will not fit here.
Resolution: For sky observation, you wish to see details and observe faint objects. People are used to increase the resolution of their cameras and screens HD, 2k, 4K etc... If it makes sense in a natural environment full of light like every day’s life, it is different for our purpose.
For deep sky, you are targeting objects that you cannot see with your eyes. They are so faint that we need to do long time exposures to capture enough light to have a picture even with CCD sensors.
A good thing for that would be to have large pixels in the camera. If so, then more photons coming from an object will be detected. But if the pixels are large, then either the sensor becomes large (and so the price) or you have a smaller resolution because less pixels per surface. Besides, the sensor should be very sensitive with a high photon ratio detection. Of course, if the pixels are too big then you will not be able to detect the faintest ones if they are too close to each other. Their photons will be merged into one pixel.
Capturing photons and storing them in CCD
Yes, but what if you want to practice pictures of bright objects like planet and sun? Of course, since you have plenty of light you can go for smaller pixels. Planetary observation requires less quality from CCD than for deep sky objects.
It is also interesting to note that on some cameras there is a mode called “Binning”. On such mode, several pixels are treated together. The photons that hits them are added and they are considered by the chip as a single pixel. You decrease the resolution but increase the signal to noise ratio.
In the end, consider the following formula:
This should be between 1 and 2.
If you are <1 you will be oversampling your capture and you will get big fuzzy stars.
If you are >2 you will be undersampling your capture and you will get square stars.
Undersampling S>2 & Oversampling<1
If you are <1 you can increase the pixel size of your camera or use a focal reducer. The limits may be subject to discussion depending on what type of objects you are targeting (planets or deep sky).
Noise : The biggest problem when you work with the limit of a sensor and try to get the most of its capabilities, is that you must deal with the electronic noise.
Every photon captured by a pixel produces an electron. It is stored in the pixel until the sensor reads from each pixel how many electrons it has. Long time exposure, allow pixels to catch more photons and convert them into electrons. But, every electronic equipment generates noise meaning electrons displacement.
Noise has several sources. Good manufacturing helps. The CCD must be very performant and generate as few noise as possible. But one the sources is temperature. The lower is the temperature, the less thermal noise you have. That’s why you see cooled cameras for astronomy. Since deep sky objects have a small signal, meaning they emit few photons in our direction, you really need a device with cooling to have a picture with as less noise as possible.
It is also possible to do some image processing to remove noise. Prior to exposure, you can take dark frames that will be used to filter the noise. In a process called stacking, you take several pictures of the object and add them. Since noise has an arbitrary distribution, it is attenuated by this operation. Nevertheless, cooling is mandatory when it comes to objects so faint that their signal might be considered as noise if the temperature is too high.
Blooming is another concern for astrophotography. As mentioned, every pixel stores the electrons created by interaction with photons until a read operation from the board that support the chip. But what if you have a bright star in your field and your exposure should be long enough to capture a fainter object elsewhere? The pixels that are exposed to the bright star may be saturated in electrons. When this happens, electrons can migrate to adjacent pixels and you get vertical bars. Of course, image processing can take care of this phenomenon but if your camera has an anti-blooming system, you’ll lose less data in processing.